We use the TN-C-S system, wherein before the GFCI you have 3-phase with a PEN that is shorted to ground when it enters the building, then it is split into PE and N wires. The GFCI is 3-phase with only 1 phase being put into the building (usually, though multiple phases aren't uncommon in larger housings). The PE wire is connected to the heating system and various other ground potential points (either to provide ground or obtain ground potential). The N wire is shorted to PE before entering the socket (or the socket itself shorts these two). Once it leaves a socket the entire thing becomes unpolarized, so to speak, so devices after the socket can't short PE and N without polarized plugs.

This avoids problems with the inrush since the inrush voltage against ground will be grounded away shortly after entering the socket (and trip your breakers). It also means that it's less likely that a single broken wire results in the entire GFCI becoming useless, though if neutral is broken it can become somewhat dangerous (but the device stops working).